1. Field of the Invention
The present invention relates to an apparatus for coding a moving picture, and particularly to a moving picture coding apparatus having a function to predict a motion vector from plural reference frames.
2. Description of the Related Art
As a basic system of band-type compression coding for a moving picture, there are a predictive coding system and a transform coding system. There is known that the video signal has statistical properties, that is, there is a correlation between pixels in a frame and between pixels between frames, and high efficiency coding is performed using the properties. The predictive coding system is a system using the correlation in the time domain, whereas the transform coding system is a system using the correlation in the frequency domain.
The predictive coding system is such that a predicted image is created by performing a motion compensated prediction from a past coded image frame (hereinafter referred to as a reference frame), and a difference signal between an image to be coded and the predicted image is coded. The transform coding system is such that what is obtained by dividing a frame into blocks for every pixels is transformed into a frequency domain by the discrete cosine transform (DCT), and an obtained transform coefficient (hereinafter referred to as a DCT coefficient) of each frequency component is quantized and is transmitted. In recent years, a system in which both of them are combined is generally adopted.
For example, in ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) recommendation H.261 or H.263, or in MPEG (Moving Picture Experts Group) by the standardization work group of image compression established under ISO (International Organization for Standardization), coding is performed in, for example, a unit of 16×16 pixel block (hereinafter referred to as a macroblock).
Further, recently, in order to further enhance the compression rate, H.264 is standardized. According to H.264, high efficiency image coding is performed by using various coding modes. For example, Impress Standard Textbook Series “H.264/AVC TEXTBOOK” supervised by Sakae Okubo, cowritten by Shinya Kakuno, Yoshihiro Kikuchi, and Teruhiko Suzuki, Aug. 11, 2004 p 82, p 119, etc. describes the feature of H.264. In this textbook, plural reference frames can be used for the motion compensated prediction, and also with respect to the pixel accuracy of the motion search, ¼ pixel accuracy higher than ½ pixel accuracy adopted in MPEG2 or MPEG4 is adopted. Accordingly, as compared with MPEG2 or MPEG4, it becomes possible to perform the motion compensated prediction with higher accuracy (see, for example, non-patent document 1).
However, as described above, in H.264, plural reference frames can be used for the motion compensated prediction, and ¼ is adopted as the pixel accuracy of the motion search. Thus, the processing amount of the motion compensated prediction is increased in proportion to the number of reference frames, and the increase in the processing amount causes increase in electric power consumption and increase in cost. This is very undesirable especially in a portable terminal in which reduction in size and weight and extension in battery lifetime are one of important problems.